The present invention relates to forming low work function layers, particularly to producing low work function surface layers on a substrate, and more particularly to a process and apparatus for forming low work function surface layers by laser ablation using short-wavelength photons.
Electron emission from sharp tips due to an applied electric field is an important phenomenon in many technical applications, such as scanning tunneling microscopy and flat panel display technology. In all such applications and, in particular, the two-referenced above, it is important that the tips have low work functions, are smooth on the nanometer scale, and are stable in varying gaseous environments and under high electric field conditions.
It is well known that alkali metal oxides or alkali-metal-silicon oxides on elemental or oxide substrates, show low work functions (˜1 eV) (see Handbook of Thermionic Properties by V. S. Fomenko, G. V. Samsonov ed., Plenum Press Data Division, New York, 1966). Layers of such materials have been applied in the prior art as pastes to be “activated” by various procedures. However, such activation procedures cannot be used for micro-meter-scale protrusions, for example, to be used as field-emitter tips.
It has been found that laser ablation can be performed on targets of arbitrary composition, even on materials of extremely high melting point, such as thorium oxide, and the deposition process is directly in line of sight from the laser impact point so that complicated structures, such as field emitter structures, can be coated at desired locations. Based on these findings, the present invention has been developed which involves a process and apparatus for producing low work function surface layers on substrates by laser ablation using short-wavelength (at or below visible wavelength) photons from low work function targets. The elemental composition of the deposited layer is controlled by the composition of the target and the gaseous environment in which the ablation process is performed.